I. Field of the Invention
The present invention relates to channel coding. More particularly, the present invention relates to a novel and improved technique for performing maximum a posteriori (MAP) decoding.
II. Description of the Related Art
xe2x80x9cTurbo codingxe2x80x9d represents an important advancement in the area of forward error correction (FEC). There are many variants of turbo coding, but most use multiple encoding steps separated by interleaving steps combined with iterative decoding. This combination provides previously unavailable performance with respect to noise tolerance in a communications system. That is, turbo coding allows communications at Eb/N0 levels that were previously unacceptably using the forward error correction techniques available at that time.
Many systems use forward error correction, and therefore would benefit from the use of turbo coding. For example, turbo codes could improve the performance of wireless satellite links, where the limited downlink transmit power of the satellite necessitates receiver systems that can operate at low Eb/N0 levels. Using turbo codes in a wireless satellite link could reduce the dish size for a digital video broadcast (DVB) system, or, alternatively, allow more data to be transmitted within a given frequency bandwidth.
Digital wireless telecommunication systems, such as digital cellular and PCS telephone systems, also use forward error correction. For example, the IS-95 over-the-air interface standard, and its derivatives, such as IS-95B, define a digital wireless communications system that uses convolutional encoding to provide coding gain to increase the capacity of the system. A system and method for processing RF signals substantially in accordance with the use of the IS-95 standard is described in U.S. Pat. No. 5,103,459 entitled xe2x80x9cSystem and Method for Generating Signal Waveforms in a CDMA Cellular Telephone Systemxe2x80x9d assigned to the assignee of the present invention and incorporated herein by reference (""459 patent).
Because digital wireless communication system like IS-95 are primarily for mobile communications, it is important to have devices that minimize power usage and which are small and light weight. Typically, this requires development of a semiconductor integrated circuit (xe2x80x9cchipxe2x80x9d) for performing most, or all, of the necessary processing. While convolutional encoding is relatively complex, the circuits necessary to perform convolutional encoding and decoding can be formed on a single chip along with any other necessary circuitry.
Turbo coding (in particular the decoding operation) is significantly more complex than convolutional encoding (and decoding). Nonetheless, it would be highly desirable to include turbo coding in digital wireless telecommunication systems, including mobile digital communication systems and satellite communication system. Thus, the present invention is directed to increasing the rate at which certain decoding operations can be performed, to facilitate the use of turbo coding in a variety of systems.
The present invention is a novel and improved technique for decoding technique with particular application to turbo, or iterative, coding techniques. In accordance with one embodiment of the invention a system for decoding includes a channel deinterleaver RAM for storing a block of symbol estimates and for reading out at least three different portions of said block in substantially simultaneous fashion. A set of 3 state metric calculators receives the three portion in a substantially simultanous fashion and generate corresponding sets of state metric calculations from said three portions of the channel deinterleaver block.